Shingle on the East Coasts of Neiv Zealand. 59 



some of the rivers. During floods, the material cut away 

 from the banks is carried down stream, and a portion is 

 deposited where the force of the current is diminished from 

 any cause. The beds of the rivers above mentioned are 

 strewn in places with stones and pebbles of all sizes. As the 

 force of the stream is reduced and becomes insufficient to 

 keep material in suspension, it is thrown down; and thus 

 it is at the mouth of the rivers the largest deposit 

 takes place. The stones and pebbles are there thrown 

 in a large mass on the shore, almost, sometimes quite, 

 blocking up the mouth, while the earth is carried 

 away into the sea. Soundings show that much stone 

 (described as gravel on the charts) is also carried into 

 the sea; but a great deal of this is doubtless thrown up 

 again on to the beach, as the author will endeavour to show 

 later on. The material is mostly a bluish-grey stone, usually 

 described as clay-slate.*" The shape of the stones, or, more 

 correctly, pebbles, may be described as flattened ovoids. 



II. — Agency by which Shingle is Caused to Travel. 



19. Material once deposited as above must, owing to its vis 

 inertice, remain till some greater force than that of the current 

 from which it was deposited comes into play. That deposited 

 in the river bed may be removed by a succeeding flood ; that 

 near the mouth of the river may be removed by a greater 

 flood than the first one ; but the disturbance by floods of 

 the material once deposited in the sea or on the seashore 

 must be limited. The action of the sea itself must be looked 

 to for any further movements of the material. In the sea 

 two forces may be considered — (1) currents, which may be 

 tidal or otherwise ; and (2) waves. Currents along a coast, 

 unless in exceptional localities, are usually of no great 

 velocity, and, consequently, able to transport the finer 

 materials only. The tidal flow into and out of an estuary, 

 creek, or lagoon may be strong, and capable of moving large 

 and heavy bodies, like a flood of similar strength in a river ; 

 but the action fails soon after the current enters the sea, in 

 the same manner as the flood of a river then loses its force. 

 With the exception of this tidal flow into and out of rivers 



* Most of the stones from both coasts show, when fractured, a silicious 

 composition. Some stones, frequently found near Napier, are evidently 

 from a sandstone formation. 



